1. Field of the Invention
The present invention relates to a method of recovering hydrogen fluoride from an aqueous solution containing ammonium fluoride and hyrdrogen fluoride and more particularly to such a method of recovery utilizing an organic liquid extractant.
2. Description of the Prior Art
Hydrofluoric acid has been commercially produced by treating fluorspar with sulfuric acid yielding hydrogen fluoride gas and calcium sulfate. The hydrogen fluoride thus produced contains varying amounts of inert contaminants such as carbon dioxide, air, water vapor, sulfur dioxide, fluosilicic acid and silicon tetrafluoride derived from silica impurities in the fluorspar. The removal of these contaminants, especially fluosilicic acid, has proved troublesome and expensive. In addition, the cost of the raw materials (low impurity fluospar and sulfuric acid) used in this process has risen in recent years.
In response to the rising cost of hydrofluoric acid production from fluorspar, a number of substitute processes have been devised. For example, the phosphate rock fertilizer industry has long been plagued by the problem of waste by-product fluorine compounds, principally fluosilicic acid and silicon tetrafluoride. These fluorine compounds comprise environmentally polluting waste products.
The industry has developed a number of processes for converting these waste fluorine compounds into marketable hydrogen fluoride. In one process, silicon tetrafluoride is sparged through water to precipitate silica and form fluosilicic acid. The fluosilicic acid thus produced is then concentrated, typically by evaporation of water and heated to produce a vapor phase containing hydrogen fluoride, silicon tetrafluoride and water. The hydrogen fluoride fraction of the vapor phase is then separated using a polyglycol extractant.
U.S. Pat. No. 3,128,152 discloses another method wherein fluosilicic acid is treated with ammonia or ammonium hydroxide to produce ammonium fluoride and solid silica. The silica is removed by any suitable means such as by filtration and the remaining ammonium fluoride solution is evaporated to crystallize out ammonium bifluoride. The ammonium bifluoride salt is then either heated until it decomposes or is oxidized at a high temperature in the presence of oxygen or air, to produce hydrogen fluoride and nitrogen. Unfortunately, these processes are economically unattractive due to the high temperatures required and because some of the ammonia is lost as nitrogen gas.
In another method, ammonium bifluoride is reacted with an alkali metal fluoride to form alkali metal bifluoride which is subsequently decomposed into hydrogen fluoride and alkali metal fluoride . The alkali metal fluoride is then recycled to the first reaction. Unfortunately, this method also suffers from high decomposition temperatures making the process economically unattractive.
Solvent extraction of hydrohalic acids from their solutions and stripping of the acids from the solvents has been described. Hardwick and Wace summarize in "Hydrofluoric Acid Recovery By Amine Solvent Extraction", Chemical and Process Engineering, June 1965, pp. 283-293, a detailed study of using undiluted tertiary amines for HF extraction from HF/water mixtures. They found that the HF extraction efficiency in an aqueous HF system was strongly dependent upon the HF concentration of the aqueous solution. They found that the amine was loaded with HF to achieve an HF:amine molar ratio of 0.05 while in equilibrium with a 0.1% HF solution and achieved an HF:amine molar ratio of 1.4 while in equilibrium with a 0.15% HF solution. As for the effect of temperature, they concluded that within the temperature range of 2.degree. to 60.degree. C. the partition coefficients were independent of temperature. The effects of amine dilution and amine strength were not studied.
Extraction of an acid from a solution of a salt of the acid has also been described. In several cases it was found that the extraction of the acid is enhanced by the presence of the salt. Thus for example, higher HCl loadings of the solvent were obtained in extraction from HCl--KCl--H.sub.2 O solutions than those obtained in extraction from HCl--H.sub.2 O solutions having similar HCl concentrations.
It is therefore an object of the present invention to provide a method of producing hydrogen fluoride from by-product fluosilicic acid without the attendant problems of the prior art processes such as high temperature decomposition and expensive raw materials.
It is another object of the present invention to provide a method of recovering hydrogen fluoride from an aqueous solution containing ammonium fluoride and hydrogen fluoride utilizing organic liquid extractants.